Tantalum catalysts for the dimerization of olefins

ABSTRACT

1-Butenes are selectively produced via the dimerization of 1-olefins in the presence of novel organotantalum catalysts comprising tantalum and a silyl-substituted cyclopentadienyl moiety having the formula C 5  H 5-x  (SiR 6   3 ) x , wherein each R 6  may be the same or different and is hydrogen, alkyl, cycloalkyl, aryl, aralkyl or alkoxy, and x is an integer from 1 to 5.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 026,358, filed Mar. 16,1987, now U.S. Pat. No. 4,717,783.

BACKGROUND OF THE INVENTION

This invention relates to tantalum compounds which are catalysts orcatalyst precursors for the dimerization of 1-olefins to 1-butenes.

Higher olefins, such as 1-butenes, find application as industrialintermediates. 2,3-Dimethyl-1-butenes, for example, are used as agasoline additive to improve octane rating and as a starting material tosynthesize musk perfumes.

The product of the dimerization of a terminal olefin is dependent on thedirection of coupling of the two olefinic units; hence, a mixture ofdimeric products is always possible. In order to achieve a high yield ofa particular dimeric product, a highly selective catalyst is required.Prior to 1980 few homogeneous catalysts were known for converting aterminal olefin selectively to 1-butene or 2,3-disubstituted-1-butenesat room temperature or above. In addition, the known catalysts readilyisomerized the initially formed 1-butenes to the thermodynamically morestable internal olefins. Moreover, some of the catalysts were activewith only one olefinic substrate. Certain nickel compounds containingphosphine ligands typify the dimerization catalysts of this period. Oneexample of said nickel compounds istris(triisopropylphosphine)nickel(O), which in the dimerization ofpropylene, yields a mixture of dimethylbutenes, methylpentenes andlinear hexenes. For a general review of this subject, see B. Bogdanovic,Advances in Organometallic Chemistry, 17 (1979), 105-140; and S.Muthukumari Pillai et al., Chemical Reviews, 86 (1986), 353-399.

More recently a series of tantalum compounds has been disclosed in U.S.Pat. Nos. 4,197,419; 4,231,947; and 4,245,131. It is further disclosedthat these tantalum compounds are capable of selectively dimerizing1-olefins, in general, to 1-butene and 2,3-disubstituted-1-butenes.These tantalum compounds contain a cyclopentadienyl group represented bythe formula C₅ H_(5-x) Me_(x), wherein Me is methyl and x is an integerfrom 0 to 5. The tantalum compounds containing this cyclopentadienylgroup and their use as catalysts in the dimerization of 1-olefins aredescribed hereinbelow.

U.S. Pat. No. 4,197,419 discloses tantalum catalyst precursors orcatalysts of the formulae ##STR1## where C₅ Me₅ ispentamethylcyclopentadienyl: X is halide or alkoxide; L is an alkenehaving from 2 to 20 carbon atoms; and R² is hydrogen or a C₁₋₁₈ alkylradical. These precursors or catalysts are disclosed to dimerize1-olefins, such as 1-propylene, 1-pentene and 1-hexene, to 1-butenesselectively. For example, it is taught that propylene is dimerized withgreater than 90 percent selectivity to 2,3-dimethyl-1-butene at a rateof about 0.039 min⁻¹ or about 1 turnover per Ta per hour at 40° C.

U.S. Pat. Nos. 4,231,947 and 4,245,131 disclose catalysts or catalystprecursors of the formula Z(R)(R¹)_(n) (R²)(A)_(m) wherein Z is tantalumor niobium: R is cyclopentadienyl or methyl-substituted cyclopentadienylhaving the formula C₅ H_(x) Me_(5-x), wherein x is an integer from 0 to5, or R is neopentylidene; R¹ is benzyl or neopentyl, n is 0 or 1; R² isneopentylidene, benzylidene, tetramethylene or2,3-dimethyltetramethylene; A is halo including chloro, bromo, iodo andfluoro or a moiety of the formula YR³ R⁴ R⁵ wherein Y is a group elementincluding N, P, Sb and Bi, and R³, R⁴ and R⁵ can be the same ordifferent and C₁₋₄ alkyl, aralkyl or aryl; and m is 1 or 2. In thedimerization of propylene by tgTa(C₅ H₅)(CHCMe₃)(Cl)₂ ], it is taughtthat 2,3-dimethyl-1-butene is produced in 93 percent selectivity at arate of 2 moles per Ta per hour at 45° C.

The preparation of the methyl-substituted cyclopentadienyl group of theaforementioned catalysts is lengthy and costly: for example, thesynthesis of lithium pentamethylcyclopentadienide is described in foursteps in Scheme 1. Commercially unavailable 2-bromo-2-butene is reactedwith lithium, and the lithiated product is condensed with ethyl acetate.The condensation product is cyclized and the resultingpentamethylcyclopentadiene ring compound is reacted with butyl lithium.These reactions give lithium pentamethylcyclopentadienide, the anion ofwhich can be introduced into a tantalum compound. ##STR2##

One skilled in the art can readily appreciate the difficulties and costof preparing lithium pentamethylcyclopentadienide by the reactionsoutlined in Scheme 1. Moreover, after the substituted cyclopentadienidegroup is prepared, the synthesis of the tantalum catalysts containingsaid cyclopentadienyl group must be accomplished. The prior art patentscited hereinabove teach the synthesis of such catalysts; but, again thesynthesis is lengthy and costly.

In view of the prior art it would be desirable to provide a catalystsystem which could be prepared easily from commercially availablestarting materials and which could find use in the selectivedimerization of 1-olefins to 1-butenes. Additionally, it would bedesirable if the easily prepared catalyst would possess a higheractivity in the dimerization of 1-olefins than the homogeneous catalystsknown heretofore.

SUMMARY OF THE INVENTION

ln one aspect, this invention is novel organotantalum compounds whichcomprise tantalum and a cyclopentadienyl group containing at least onetri-substituted silyl moiety. The silyl-containing cyclopentadienylgroup of this invention can be easily prepared from commerciallyavailable starting materials. Consequently, this invention providesorganotantalum compounds which require less effort to synthesize thanthe organotantalum compounds of the above-cited prior art patents.

In another aspect, this invention is a process for the selectivedimerization of a 1-olefin to a dimer having a 1-butene moiety which isoptionally substituted at the 2,3-carbon atoms, said process employingthe novel organotantalum compounds as catalysts. The process of thisinvention comprises contacting a 1-olefin with an organotantalumcatalyst under reaction conditions such that a dimer having a 1-butenemoiety which is optionally substituted at the 2,3-carbon atoms isformed; said catalyst comprising tantalum and a cyclopentadienyl groupcontaining at least one tri-substituted silyl moiety. The tantalumcatalysts of this invention can be advantageously used in thedimerization of any one of a plurality of unsubstituted or substituted1-olefin feed compositions. Surprisingly, the tantalum catalysts of thisinvention exhibit a higher activity towards the dimerization of1-olefins than the tantalum dimerization catalysts known heretofore.

DETAILED DESCRIPTION OF THE INVENTION

The novel compounds of this invention are organotantalum compoundscomprising tantalum and a cyclopentadienyl group containing at least onetri-substituted silyl moiety. Preferred novel compounds of thisinvention are represented by one of the formulae: ##STR3## wherein X isa halide, including chloride, bromide, iodide and fluoride, or analkoxide; L is an alkene having from 2 to 20 carbon atoms and which canbe substituted with alkyl or aryl, and Cp^(s),x is a substitutedcyclopentadienyl group, defined hereinafter; ##STR4## wherein X isdefined in (I) above, R^(o) is hydrogen or a C₁₋₁₈ alkyl, such as ethyl,propyl or butyl, and Cp^(s),x is defined hereinafter; and wherein R¹ isbenzyl or neopentyl; n is 0 or 1; R² is neopentylidene or benzylidene; Ais halide or a moiety of the formula YR³ R⁴ R⁵ wherein Y is a Group Vaelement, including N, P, Sb and Bi, and R³, R⁴ and R⁵ can be the same ordifferent and are C₁₋₄ alkyl, aralkyl such as benzyl, neopentyl, toluylor xylyl, aryl such as phenyl, naphthyl or bipyridyl; m is 1 or 2; andCp^(s),x is defined hereinafter.

Cp^(s),x represents cyclopentadienyl having at least one tri-substitutedsilyl moiety, such that Cp^(s),x has the formula C₅ H_(5-x) (SiR⁶₃)_(x), wherein each R⁶ is the same or different and is hydrogen; C₁₋₂₀alkyl, such as methyl, ethyl, isopropyl; cycloalkyl, such as cyclohexyl;aryl, such as phenyl or toluyl; aralkyl, such as benzyl; or alkoxy, suchas methoxy or ethoxy; and x is an integer from 1 to 5.

The novel tantalum catalysts, described hereinbefore, may be prepared ina two-part synthesis. The first part consists in synthesizing theCp^(s),x group; the second part consists in synthesizing the noveltantalum compounds containing the Cp^(s),x group.

The Cp^(s),x compounds are easily prepared from commercially availablecyclopentadiene metal salts, M(C₅ H₅). Lithium cyclopentadienide orsodium cyclopentadienide is a suitable starting material. The metal saltis reacted with a halo tri-substituted silane having the formula R⁶ ₃SiX, wherein R⁶ and X are defined hereinbefore, to yield cyclopentadienecontaining at least one tri-substituted silyl moiety. Preferably, R⁶ isa C₁₋₂₀ alkyl, aryl, or aralkyl moiety. More preferably, R⁶ is a C₁₋₄alkyl moiety. Most preferably, R⁶ is a methyl moiety. Preferably, X ischloro. Examples of some commercially available halo tri-substitutedsilanes which may be used in the synthesis of Cp^(s),x arechlorotrimethylsilane, chlorotriethylsilane, chlorotriisopropylsilane,chlorotributylsilane, chlorotribenzylsilane, chlorotriphenylsilane,bromotrimethylsilane, bromotriethylsilane, iodotrimethylsilane,chlorosilane, bromosilane, chlorotrimethoxysilane andbromotriethoxysilane. The most preferred R⁶ ₃ SiX ischlorotrimethylsilane.

The reaction of the cyclopentadienide metal salt with the halotri-substituted silane is conducted under conditions such thatcyclopentadiene containing at least one tri-substituted silyl moiety isobtained. Typically, the reaction temperature may be in the range 0° C.to 40° C., while the pressure is autogenous.

The tri-substituted silyl cyclopentadiene product has the formula tgC₅H_(5-x) (SiR⁶ ₃)_(x) ], wherein R⁶ and x are defined hereinabove. Thedegree of substitution on the cyclopentadiene ring will depend on themole ratio of halo tri-substituted silane to unsubstitutedcyclopentadienide. Thus, if said ratio is one, the cyclopentadieneproduct will be mono substituted (x=1). lf said ratio is two, thecyclopentadiene product will be disubstituted (x=2); and so on. In thoseinstances where x is 2 or greater, the substitution of each silyl moietyonto the cyclopentadienide ring is preferably carried out sequentially.For example, in order to prepare bis(tri-substitutedsilyl)cyclopentadiene, the mono derivative is converted to thecorresponding cyclopentadienide metal salt by reaction, for example,with butyl lithium. The metal salt is reacted with a second mole of halotri-substituted silane to obtain the desired bis(tri-substitutedsilyl)cyclopentadiene in which x=2. The preferred cyclopentadieneproduct contains two tri-substituted silyl moieties (x=2). The morepreferred cyclopentadiene product contains two trimethylsilyl moietiesand has the formula 1,3-tgC₅ H₄ (SiMe₃)₂ ].

In order to prepare the tantalum catalysts containing thesilyl-substituted cyclopentadiene compound, said compound must beconverted into the corresponding metal cyclopentadienide salt. It isfrom the salt form that the cyclopentadiene compound is transferred to atantalum compound. Thus, in the final step, the cyclopentadiene compoundcontaining the tri-substituted silyl moieties is reacted with butyllithium to give LiCp^(s),x which has the formula Li[C₅ H_(5-x) (SiR⁶₃)_(x) ], the Cp^(s),x of which can be incorporated into a tantalumcompound.

The novel tantalum catalysts of this invention can be prepared bymethods which are similar to those previously disclosed in U.S. Pat.Nos. 4,231,947 and 4,197,419. For example, compounds of Formula I areprepared by reacting TaX₅ with Zn(CH₂ SiMe₃)₂, wherein X and Me arehalide and methyl, respectively, at a temperature of between about 20°C. and 40° C. in toluene or pentane to give Ta(CH₂ SiMe₃)X₄. The latteris reacted with LiCp^(s),x at a temperature of between 20° C. and 40° C.in toluene or diethyl ether to give TaCp^(s),x (CH₂ SiMe₃)X₃. SaidTaCp^(s),x (CH₂ SiMe₃)X₃ is reacted with one-half mole of Zn(CH₂ CH₂CH₃)₂ to give a compound of Formula I where L is propylene. Any otherolefin, L, may be substituted for propylene by merely exposingTaCp^(s),x (CH₂ CHCH₃)X₂ to the desired olefin overnight at 25° C.Compounds of Formula II are prepared by equilibrating a compound ofFormula I with an excess of olefin L.

The preparation of Formula III compounds will vary depending on theligands involved. TaCp^(s),x (CHCMe₃)Cl₂, for example, can be preparedby reacting TaCl₅ with Zn(CH₂ CMe₃)₂ at a temperature between about 20°C. and 40° C. in toluene or pentane to form Ta(CH₂ CMe₃)2Cl₃. The lattercompound is then reacted with LiCp^(s),x at a temperature of betweenabout 20° C. and 40° C. in toluene or diethyl ether to form the desiredproduct TaCp^(s),x (CHCMe₃)Cl₂. Other representative compounds ofFormula III encompassed by this invention include; ##STR5##

As described hereinbefore, the novel tantalum compound may contain aGroup V donor having the formula YR³ R⁴ R⁵. Examples of Group V donorswhich are suitable groups to incorporate into the novel tantalumcompounds are the following: PMe₃, PMe₂ Ph, PPH₃, NMe₃, NEt₃, AsMe₃,BiMe₃, P(OMe)₃, P(OPh)₃ wherein Me is methyl, Et is ethyl and Ph isphenyl. Optically active phosphines of the formula PR³ R⁴ R⁵ are alsosuitable.

The novel tantalum catalysts of this invention are homogeneous in thatthey are in the same phase as the reactants under the conditionsemployed in the process. It is within the scope of this invention,however, for the tantalum catalysts to be used in a heterogeneous formby binding or physically adsorbing said catalyst onto a solid support.Such supports can be any material, providing it does not interfere withthe dimerization reaction. Refractory oxides provide suitable supports.Such refractory oxides include alumina, silica, zirconia, boria,magnesia, titania, tantala, kieselguhr and mixtures of these materials.The support material can also be an activated carbon or a polymer, suchas polystyrene. The support may be characterized by any pore size orpore shape, and may have any surface area, providing the support doesnot inhibit the activity of the tantalum catalyst. A description ofsuitable supports and their distinguishing features may be found inHeterogeneous Catalysis in Practice by C. N. Satterfield, (1980) atpages 86-94; and in "Polymer Supported Catalysts" by C. U. Pittman, Jr.in Comprehensive Organometallic Chemistry, G. Wilkinson, F. G. A. Stone,and E. W. Abel, eds., (1982), Volume VIII, at pages 553-611; and isincorporated herein by reference.

The supports disclosed hereinbefore may be chemically reacted with atantalum catalyst precursor to give the tantalum catalyst chemicallybonded to the support. Alternatively, the support may be treated withthe tantalum catalyst or catalyst precursor, so as to obtain a tantalumcatalyst physically adsorbed onto the support. The preparation of saidsupported catalysts is carried out by methods well-known in the art, forexample, incipient wetness impregnation techniques and precipitationtechniques, as described in detail in Heterogeneous Catalysis inPractice, by C. N. Satterfield, (1980) at pages 70-86.

In the practice of the method of the present invention, any one of aplurality of unsubstituted or substituted 1-olefin feed compositions maybe employed in the dimerization reaction. In this context, theunsubstituted 1-olefin is defined as an acyclic aliphatic hydrocarbonhaving one double bond at the alpha, or first, carbon. The 1-olefin maybe substituted with certain moieties, providing these moieties do notoccupy a position on the doubly-bonded carbon atoms, namely the α,β or1,2 carbons. It is desirable for these moieties to occupy a position atthe third carbon or any carbon further along the chain. Those moietieswhich are suitable substituents will not interfere with the dimerizationreaction. Such moieties include C₁₋₂₀ alkyl, such as methyl, ethyl orisopropyl: cycloalkyl, such as cyclohexyl: aryl, such as phenyl;aralkyl, such as benzyl: alkoxy, such as methoxy or ethoxy: and halo,such as chloro, bromo or iodo groups. It is desirable for the alkoxy andhalo moieties to be located beyond the 4-carbon position along thechain. Examples of 1-olefins suitable for use in the dimerizationreaction include the following: ethylene, propylene, 1-butene,1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene,1-dodecene, 3-methyl-1-butene, 3-methyl-1-pentene, 4-methyl-1-pentene,3-methyl-1-hexene, 4,4-dimethyl-1-pentene and 3,3-dimethyl-1-butene.Preferably, the 1-olefin is a C₁₋₁₂ 1 -olefin. More preferably, the1-olefin is a C₁₋₈ 1 -olefin. Most preferably, the 1-olefin ispropylene.

The tantalum catalyst is added to the 1-olefin in an amount which issufficient to catalyze the dimerization reaction. Preferably, the weightpercent of tantalum catalyst relative to 1-olefin substrate is in therange from about 0.01 weight percent to about 50 weight percent. Belowthe preferred lower limit, recovery of the catalyst becomes difficultand the efficiency of the reaction is too low. Above the preferred upperlimit, the capacity of the reactor may be reduced, the product may bedifficult to purify, and the economics may be disadvantageous. Morepreferably, the weight percent of the tantalum catalyst relative to1-olefin is in the range from about 0.1 to about 5 weight percent.

The dimerization reaction may be carried out neat, if the 1-olefinsubstrate is a liquid under the process conditions or in a solvent, asdesired. The solvent can be any compound which solubilizes the 1-olefinsubstrate, the dimerization products and the catalyst, and which isinert under the process conditions. Typical solvents include alkanes,such as pentane or hexane; aromatic compounds, such as benzene ortoluene; chlorinated aliphatic or aromatic compounds, such as methylenedichloride or chlorobenzene. The weight ratio of solvent relative to1-olefin substrate may vary from 0:1 to about 100:1: however, preferablythe weight ratio is 0:1 to about 10:1.

Process conditions may vary over a wide range. Any reaction temperatureand pressure may be maintained providing the 1-olefin feedstock isconverted to the desired 1-butene or 2,3-disubstituted-1-butenedimerization product. The reaction temperature preferably ranges fromabout -30° C. to about 200° C. Below -30° C., the reaction is slow andthe reaction time is inordinately long. Above 200° C., the productselectivity is reduced. The upper preferred limit may also be restrictedby the boiling points of the reactants, solvents and products, and bythe equipment at hand. More preferably, the reaction temperature rangesfrom about 25° C. to about 125° C. The reaction pressure preferablyranges from about 0.019 psig (1 mm Hg) to about 1000 psig. The preferredlimits of pressure depend primarily on convenience and cost. Outside thestated limits costs increase and ease of operation decreases. Morepreferably, the reaction pressure ranges from about 14 psig to about 200psig.

In accordance with the present invention, the dimerization of a 1-olefinto a dimer having a 1-butene moiety which is optionally substituted atthe 2,3-carbon atoms may be conducted in any conventional reactordesigned to withstand the pressure of the reaction. Thus, at pressuresgreater than atmospheric standard pressure, reactors, such as pressurebottles or metal autoclaves, may be employed. The reactor may beequipped with a means for controlling temperature, a means for measuringtemperature and a means for agitating the reaction mixture.

For the purposes of the dimerization process of the present invention,the term "selectivity" refers to the mole percentage of the converted1olefin which goes to a particular dimeric product. A dimer having a1-butene moiety which is optionally substituted at the 2,3-carbon atomsis the major dimeric product of the process. Typically, a selectivity ofat least about 70 mole percent is achieved in the production of saiddimer. Preferably, the selectivity to a dimer having a 1-butene moietywhich may be substituted at the 2,3-carbon atoms is at least about 80mole percent; more preferably, the selectivity is at least about 90 molepercent; most preferably, the selectivity is at least about 95 molepercent.

For the purposes of the dimerization process of the present invention,the term "catalyst activity" refers to the rate at which the catalystconverts the 1-olefin to dimeric products. This rate is measured inunits of moles of 1-olefin consumed per molar concentration of tantalumcatalyst per hour, (mol. M⁻¹ hr.⁻¹). The catalyst activity is dependentupon the temperature of the reaction and the specific 1-olefin employed.Typically, the catalyst activity is at least about 1.36 mol. M⁻¹ hr.⁻¹at 75° C., and at least about 4 mol. M⁻¹ hr.⁻¹ at 100° C. Preferably,the catalyst activity is at least about 1.36 mol. 1-octene M⁻¹ hr.⁻¹ at75° C.; 4.0 mol. 1-octene M⁻¹ hr.⁻¹ at 100° C.; 2.0 mol. propylene M⁻¹hr.⁻¹ at 75° C.; and 12.3 mol. propylene M⁻¹ hr.⁻¹ at 98° C.

ILLUSTRATIVE EMBODIMENTS

The following examples and comparative experiments will serve toillustrate the invention, but are not meant to limit the scope therein.

EXAMPLE 1 Preparation of TaCp^(s),2 Cl₂ (cyclooctene) A. Preparation ofLi{C₅ H₃ (SiMe₃)₂ }

Trimethylsilylcyclopentadiene and bis(trimethylsilyl)cyclopentadiene areprepared by the procedures of Kraihanzel et al., J. Amer. Chem. Soc.,90, 4701 (1968) and I. M. Pribytkova et al., J. of Organometallic Chem.,30, C57-C60 (1971). Sodium cyclopentadienide (44 g, 0.50 mole) is addedto 150 ml of tetrahydrofuran. Chlorotrimethylsilane (54 g, 0.5 mole) isadded dropwise slowly to the reaction and the stirring is continued for3 hours. The reaction mixture is poured into 150 ml of cold, distilledwater and trimethylsilylcyclopentadiene is extracted into diethyl ether.The extract is rotary evaporated to remove the ethereal solvent and theresidue is vacuum distilled to give trimethylsilylcyclopentadiene, b.p.41° C.-43° C. (16 mm).

Freshly distilled trimethylsilylcyclopentadiene (1.6 g, 11.5 mmole) isdissolved in 10 ml of absolute ligroin. To this solution is added asolution of butyllithium (1.9N, 6.35 ml, 12.0 mmole) in ligroin dropwisewith stirring. The mixture is stirred for one hour at room temperature.Chlorotrimethylsilane (2.6 g, 24 mmole) is added to the mixture under anargon sweep, and the resulting mixture heated for 3 hours at 40° C. Theligroin solution is filtered, the solvent evaporated, and the residuedistilled under a vacuum to give 1,3-bis(trimethylsilyl)cyclopentadiene,b.p. 45° C./3 mm Hg.

1,3-Bis(trimethylsilyl)cyclopentadiene (10.5 g, 0.05 mole) is reactedwith 32 ml of a 1.6M solution of butyllithium (0.05 mole) in hexane togive 6.3 g of lithium 1,3-bis(trimethylsilyl)cyclopentadienide.

B. Preparation of Catalyst

TaCp^(s),2 (CH₂ SiMe₃)Cl₃ is prepared by the method of P. A. Belmonte etal., J. Amer. Chem. Soc., 105, 2643 (1983). TaCp^(s),2 (CH₂ SiMe₃)Cl₃(500 mg, 0.856 mmole) is dissolved in 30 ml of toluene. The solution iscooled to -30° C. under an atmosphere of argon. Zn(CH₂ CH₂ CH₃)₂ (71.3mg, 0.47 mmole) is added with stirring. The orange-colored solutiondarkens and a small amount of precipitate forms. The reaction mixture isplaced in a refrigerator at -30° C. for 30 minutes, after which it isremoved to an inert atmosphere and allowed to warm to room temperaturewith stirring. When the color of the solution is changed to deep purple,after approximately 2 hours, the solution is filtered. Cyclooctene (1 g,0.91 mmole) is added to the filtrate and the reaction mixture is allowedto stand overnight at 25° C. The solvent is evaporated under reducedpressure (20 mm Hg) and the purple-black solid is recrystallized frompentane. {Ta[C₅ H₃ (SiMe₃)₂ ](Cl)₂ (cyclooctene) } (420 mg, 86 percentyield) is obtained as black crystals.

The 90 MHz 1H nuclear magnetic resonance spectrum of {Ta[C₅ H₃ (SiMe₃)₂](Cl)₂ (cyclooctene)} in benzene-d₆ shows the cyclopentadiene ringprotons at τ4.44 to 4.14 (split triplet, 3), the cyclooctene protons atτ8.57 (broad singlet), and the protons of the trimethyl silyl groups atτ9.90 (singlet, 18).

EXAMPLES 2(a)-(c) Dimerization of 1-Octene

All liquid reagents are first deoxygenated in three freeze-pump-thawcycles, then dried over molecular sieves, and dried again by passingthrough a column of activated alumina. 1-Octene is additionally treatedto prevent catalyst deactivation. The additional pretreatment involveswashing the 1-octene with aqueous ferrous sulfate twice; then drying thewashed 1-octene over MgSO₄ ; deoxygenating the dried 1-octene in threefreeze-pump-thaw cycles; and finally drying the deoxygenated 1-octeneover molecular sieves under an inert gas atmosphere.

Example 2(a) is carried out as follows: TaCp^(s),2 Cl₂ (cyclooctene)(150 mg, 0.263 mmole) is dissolved in 2.7 ml. of toluene in a 30-ml.screw cap vial in a drybox under an atmosphere of argon. Decane (150 μl,109.5 mg, 0.77 mmole) is added to the solution for use as an internalgas phase chromatography standard. 1-Octene (1.57 g, 14 mmoles) is addedto the solution. The reaction mixture is brought to the desiredtemperature by means of an oil bath. Aliquots are periodically removedfrom the reactor for analysis by capillary gas phase chromatography on a25-meter, 5 percent phenylmethylsilicone column (flow rate of 1 ml/minHe at 100/1 split injection, temperature programmed at 60° C. for 4minutes rising to 180° C. at 16° C./minute).

Examples 2(b) and 2(c) are carried out in a manner analogous to Example2(a). The results of Examples 2(a)-(c) are presented in Table I.

The predominant product is identified to be 2-hexyl-3-methyl-1-nonene,which is designated the tail-to-tail (tt) dimer. The secondary productis identified to be 2-hexyl-1-decene, which is designated thehead-to-tail (ht) isomer. Other isomers, including internal olefinicdimers, are present in low yield.

                  TABLE I                                                         ______________________________________                                             1-        Catalyst        Selectivity*                                   Ex.  Octene    Solvent  T      (mole %)                                       2    (g)       (mg/ml)  (°C.)                                                                         tt   ht     others                             ______________________________________                                        (a)  1.57      150/2.7  50     83.7 15.9   2.4                                (b)  3.14      109/5.4  75     77.0 16.9   6.1                                (c)  3.14       16/5.4  100    78.3 20.5   6.0                                ______________________________________                                         *tt is the tailto-tail dimer, 2hexyl-3-methyl-1-nonene                        ht is the headto-tail dimer, 2hexyl-1-decene others, include internal         olefinic dimers                                                          

The tail-to-tail dimer is obtained in a selectivity of greater than 70percent under mild process conditions. At 50° C. the rate is found to be0.08 mol. M⁻¹ hr.⁻¹. At 75° C. the rate is found to be 1.36 mol. M⁻¹hr.⁻¹, and at 100° C. the rate is found to be 3.96 mol. M⁻¹ hr.⁻¹.

COMPARATIVE EXPERIMENT 1 A. Preparation of Ta(C₅ Me₅)Cl₂ (cyclooctene)

All operations are conducted under an atmosphere of argon. Ta(C₅Me₅)(CH₂ CMe₃)Cl₃ (422 mg, 0.856 mmole), wherein C₅ Me₅ ispentamethylcyclopentadienyl, and CH₂ CMe₃ is neopentyl, is dissolved in20 ml of toluene. The solution is cooled to -78° C. and Zn(CH₂ CH₂ CH₃)₂(71 mg, 0.47 mmole) dissolved in 5 ml of toluene is added all at once.The reaction mixture is stirred for 15 minutes and then warmed to roomtemperature. A precipitate of ZnCl₂ is filtered off and 300 mg ofcyclooctene is added. The reaction mixture is left to stand overnight at25° C. Deep purple crystalline product (260 mg, 61 percent) isrecovered. This product is recrystallized from toluene-pentane, andidentified as Ta(C₅ Me₅)Cl₂ (cyclooctene) from the ¹ H nuclear magneticresonance spectrum. ¹ H nuclear magnetic resonance (τ, C₆ D₆) 6.90-9.20with a broad singlet at 7.33 (m, cyclooctene) and 8.39 (s, C₅ Me₅).

B. Dimerization of 1-Octene

The general procedure of Example 1 is followed. Ta(C₅ Me₅)Cl₂(cyclooctene) (177 mg, 0.356 mmole) is dissolved in 2.66 ml of toluenein a 30-ml screw cap vial. Decane (150 μl, 109.5 mg, 0.77 mmole) isadded to the solution for use as an internal gas phase chromatographystandard. 1-Octene (1.57 g, 14 mmoles) is added to the solution. Thereaction mixture is brought to 50° C. in an oil bath. The reaction isanalyzed by the method described in Example 1 and found to containtail-to-tail dimer 2-hexyl-3-methyl-1-nonene in a selectivity of 84 molepercent. The catalyst activity is found to be 0.046 mol. M⁻¹ hr.⁻¹ at50° C.

When Example 2(a) is compared with Comparative Experiment 1, it is seenthat the trimethylsilyl-substituted catalyst TaCp^(s),2 Cl₂(cyclooctene) is about 1.7 times more active at 50° C. than Ta(C₅Me₅)Cl₂ (cyclooctene), while maintaining a comparable selectivity.

EXAMPLES 3(a)-(d) Dimerization of Propylene

Example 3(a) is carried out as follows: All liquid reagents aredeoxygenated in three freeze-pump-thaw cycles, then dried over molecularsieves. TaCp^(s),2 Cl₂ (cyclooctene) (210 mg, 0.399 mmole) is dissolvedin 40 ml of toluene. Decane (1.8 ml, 1.31 g, 9.2 mmoles) is added to thesolution for use as an internal gas phase chromatography standard. Thereactor is charged at room temperature with propylene (Matheson, 99.0percent grade) to a pressure of 150 psig. The temperature is raised tothe desired temperature by means of an oil bath. Aliquots areperiodically removed from the reactor for analysis by capillary gasphase chromatography, as described in Example 1 (except that thetemperature program is 40° C. for 4 minutes rising to 150° C. at 32°C./minute).

Examples 3(b), (c), and (d) are conducted in a manner analogous to thatdescribed for Example 3(a). The results of Examples 3(a), (b), (c), and(d) are presented in Table II.

                  TABLE II                                                        ______________________________________                                               Catalyst/                                                              Ex.    Solvent    T      Selectivity* (mole %)                                3      (mg/ml)    (°C.)                                                                         tt      ht   others                                  ______________________________________                                        (a)    210/40     53     95           1.8                                     (b)    100/30     73     94      2.6  3.3                                     (c)    228/30     80     94      2.8  3.9                                     (d)    210/40     98     91      3.5  6.1                                     ______________________________________                                         *tt is the tailto-tail dimer 2,3dimethyl-1-butene                             ht is the headto-tail dimer 2methyl-1-pentene others, include internal        olefinic dimers                                                          

The catalyst activity is found to be the following: (a) 0.8, (b) 2.0,(c) 4.88 and (d) 12.3 mol. M⁻¹ hr.⁻¹. The tail-to-tail dimer is obtainedin a selectivity of greater than 90 percent under mild processconditions.

COMPARATIVE EXPERIMENT 2

The general procedure of Example 2 is followed. Ta(C₅ Me₅)Cl₂(cyclooctene) (200 mg, 0.402 mmole), wherein C₅ Me₅ ispentamethylcyclopentadienyl, prepared as in Comparative Experiment 1A,is dissolved in 30 ml of toluene. Decane (1.8 ml, 1.3 g, 9.2 mmoles) isadded to the solution for use as an internal gas phase chromatographystandard. The reactor is charged at room temperature with propylene to apressure of 140 psig. The temperature of the reactor is brought up tobetween 80° C. and 90° C. with stirring. Samples are removedperiodically for analysis, as in Example 2, and are found to contain2,3-dimethyl-1-butene in a selectivity of 96.1 mole percent. Theactivity of the catalyst is measured at 2.7 mol. M⁻¹ hr.⁻¹ at 85° C.

When Example 3(c) is compared with Comparative Experiment 2, it is seenthat the trimethylsilyl-substituted catalyst TaCp^(s),2 Cl₂(cyclooctene) is about two times more active at 80° C.-85° C. than Ta(C₅Me₅)Cl₂ (cyclooctene), while maintaining comparable selectivity.

What is claimed is:
 1. The catalyst represented by the formula: ##STR6##wherein X is halide or alkoxide, R^(o) is hydrogen or a C₁₋₁₈ alkyl, andCp^(s),x is a cyclopentadienyl group containing at least onetri-substituted silyl moiety; said cyclopentadienyl group beingrepresented by the formula C₅ H_(5-x) (SiR⁶ ₃)_(x), wherein each R⁶ isthe same or different and is hydrogen, alkyl, cycloalkyl, aryl, aralkylor alkoxy, and X is an integer from 1 to
 5. 2. The catalyst of claim 1wherein X is chloride.
 3. The catalyst of claim 1 wherein R^(o) ishydrogen.
 4. The catalyst of claim 1 wherein R^(o) is ethyl.
 5. Thecatalyst of claim 1 wherein Cp^(s),x is 1,3-[C₅ H₃ (SiMe₃)₂ ] wherein Meis methyl.
 6. The catalyst represented by the formula: ##STR7## whereinR¹ is benzyl or neopentyl; n is 0 or 1; R² is neopentylidene orbenzylidene; A is halide or a moiety of the formula YR³ R⁴ R⁵ wherein Yis a Group Va element and R³, R⁴, and R⁵ are the same or different andare C₁₋₄ alkyl, aralkyl or aryl, m is 1 or 2, and Cp^(s),x is acyclopentadienyl group containing at least one tri-substituted silylmoiety; said cyclopentadienyl group being represented by the formula C₅H_(5-x) (SiR⁶ ₃)_(x), wherein each R⁶ is the same or different and ishydrogen, alkyl, cycloalkyl, aryl, aralkyl or alkoxy, and x is aninteger from 1 to
 5. 7. The catalyst of claim 6 wherein A is a halide.8. The catalyst of claim 6 wherein A is a moiety of the formula YR³ R⁴R⁵.
 9. The catalyst of claim 8 wherein Y is N.
 10. The catalyst of claim8 wherein Y is P.
 11. The catalyst of claim 6 wherein Cp^(s),x is1,3-[C₅ H₃ (SiMe₃)₂ ] wherein Me is methyl.
 12. The catalyst of claim 6wherein R¹ is neopentyl.
 13. The catalyst of claim 6 wherein R² isneopentylidene.